This invention relates to a method for the preparation of an electrocatalyst which may be utilized as an electrode for an electrochemical cell as described in U.S. Pat. No. 3,651,386.
An electrochemical cell is basically comprised of an anode and a cathode positioned in an electrolyte and connected in an external circuit, although many variations of the physical arrangement of the three components are possible. An electrochemical cell is a device which permits the performance of oxidation or reduction reactions electrochemically, that is, by way of an electron transfer reaction at an electrode-electrolyte interface. Oxidation reactions take place at the anode while reduction reactions take place at the cathode.
Electrochemical cells can be classified according to their use. Some produce energy and are called batteries. Others are used to produce chemicals under the use of energy and are called electrolysis cells.
There are a great many different types of energy producing electrochemical cells, such as primary batteries, secondary batteries, fuel cells and batteries which are combinations where one electrode may be a fuel cell electrode, the other a conventional battery electrode, such as is the case in the zinc-air battery.
If the cell is a fuel cell, fuel is supplied from an external source to the anode where it is oxidized, thereby freeing electrons which flow in the external circuit. The oxidation of the fuel also results in the production of hydrogen ions at the anode. These hydrogen ions pass through the electrolyte to the cathode, where they combine with oxygen and electrons to form water. Electrodes of a fuel cell may be of the diffusion type, and usually are porous and have at least one surface impregnated with a catalyst, such as the catalyst substance of this invention. Chemical and catalytic action takes place only at the interface between the electrolyte, the reacting gas, and an electrode.
As it is desirable to design an electrochemical cell so as to increase the surface of this interface, the electrodes are often constructed with at least one surface of a porous material and with a hollow interior. The reacting fuel gas and the oxygen are forced into the interior of the pores of the respective electrodes where the gases meet the electrolyte. The electrochemical reactions take place at a three phase boundary area. It is at this boundary area of the anode or cathode that oxidation of the fuel and reduction of the oxygen takes place, thereby producing electricity in the external circuit, and it is this boundary area that has to have catalytic activity.
Fuel cells are often classified on the basis of their mode of operation. Typical high temperature fuel cells which operate at 800.degree. to 1200.degree. C. use solid electrolytes and gaseous fuels. Molten salt electrolytes are used in fuel cells operating at temperatures from 400.degree. to 800.degree. C. They use gaseous fuels also. Low temperature fuel cells operate at temperatures from ambient to 200.degree. or 300.degree. C. use liquid, dissolved or gaseous fuels. The oxidizing agent in most fuel cells is air, although others such as chlorine gas may be used as well. The range of available fuels is much larger. Examples are hydrogen, alcohols, hydrazine, hydrocarbons, and many more. The power which can be obtained from a battery is given by the current which can be drawn under a given voltage. It is characteristic of all chemical energy conversion devices that the voltage difference between the anode and cathode decreases as the current goes up. This voltage decrease is called polarization. Since one always attempts to obtain highest power output possible, one is constantly striving to reduce the polarization of the fuel cell electrodes. This is achieved by increasing the temperature of operation or by the use of an electrocatalyst such as is claimed in this invention.
The electrodes are often composed of a structural base section and a catalyst material mounted on the base. The structural base section usually takes the form of conductive screens or gauzes. The electrode is held in place by an electrically conductive holder having an opening. It is upon this opening that the electrode is mounted. The holder is made of electroconductive material, such as copper, silver, carbon and the like. The holder is directly conducted to the electric terminal of the external circuit and is hollow with an inlet opening through which fuel or oxygen (air) may be supplied to one side of the electrode. The electrode assembly is located below the surface of the electrolyte such that the other surface of the electrode is in contact with this electrolyte.
A typical gas diffusion electrode used in the manner described above permits the fuel gas or oxygen or air to diffuse into the interior of the pores of the electrode from one side while the electrolyte penetrates the pores from the electrolyte side. In this manner, an extended area or interface for three phase contact is achieved.
This is often brought about by incorporating a certain hydrophobicity to the electrolyte by compacting the catalyst material with a hydrophobic powderous plastic material or by such techniques as spraying one surface with a solution of Teflon, oil, or other polymeric materials, or any other suitable means. Appropriate plastic polymers include porous polytetrafluoroethylene, porous polyethylene, porous polyurethane foams, polystyrene, cellophane, polyvinylidene chloride, polyvinyl chloride, polyvinyl ethyl ether, polyvinyl alcohol, polyvinyl acetate, polypropylene cellulose, polymethyl methacrylate, butadiene-styrene copolymers, styrenated alkyd resins, some poly-epoxide resins, and chlorinated rubber.
The success of an electrochemical cell using a catalyst is fundamentally measured by the cost of producing electricity in the cell. Factors which are determinative of this cost include the temperature at which, for example, a fuel cell must be maintained during operation, the coulombic efficiency at which the fuel is oxidized, the cost of the fuel used, the cost of the catalyst used, and the life or stability of the catalyst, and finally the thermodynamic efficiency.
An important object of fuel cell development is to obtain high discharge voltage at current rates which produce a good watt/pound ratio. This can be achieved if the current-voltage characteristic of the electrode is close to the theoretical Tafel slope and exhibits a minimum of overvoltage.
The prior art has disclosed various fuel cell electrode catalysts which may be used in an electrochemical cell. For example U.S. Pat. No. 3,857,737 discloses a fuel cell electrode catalyst comprising a noble metal catalyst such as platinum deposited on particles of an inert carrier such as carbon, the catalyst being prepared by admixing the carbon powder with a salt of platinum to form a slurry followed by concentration and drying. Likewise, U.S. Pat. No. 3,364,074 discloses a carbon containing electrode which is contacted with an organic solution containing a wetproofing agent for the electrode and an organometallic compound, the electrode then being heated at a temperature sufficient to decompose the organic portion of the organometallic compound to form the desired electrode. Another U. S. patent which discloses an electrochemical cell in U.S. Pat. No. 3,881,957 in which a support such as an inorganic refractory oxide may be preimpregnated with a metal and thereafter the inorganic refractory oxide containing a coating of the catalytic metal is heated in an atmosphere containing an organic pyrolyzable material whereby a pyropolymer is deposited on the surface of the support. However, the electrocatalyst thus prepared possesses a drawback or defect in that the temperature which is required to pyrolyze the organic pyrolyzable substance is of the magnitude of from about 400.degree. to about 900.degree. C., the preferred range being from about 850.degree. to about 900.degree. C. The use of a temperature of this magnitude wil agglomerate the metal crystallites and increase the size of the crystal. This increased crystal size may be deleterious to the function of the electrocatalyst due to the fact that the surface of the catalytic metal will be minimized and will therefore decrease the activity of the electrocatalyst. As will hereinafter be shown in greater detail, in contradistinction to this method of preparing an electrocatalyst, the process of the present invention will permit the preparation of an electrocatalyst wherein the catalytic metal is impregnated on the surface of the carbonaceous pyropolymer at temperatures which will not disturb the crystallite size of the metal and therefore the crystallite size will remain in a desired range.